Hi, I have designed and built a 100 watt EL34 push pull power amp for a guitar and I am having problems with the volume and presence controls. When The presence is on maximum there is a buzzing heard through the speakers. Also when the volume control is on minimum there is the same sound.
Interestingly if the presence has recently 'buzzed', the volume control will stay silent, but if the amplifier has been left for a few minutes, moving the volume to zero will make noise. Can anybody offer any solutions to this problem because it is baffling me!
The only way these controls are connected is to a star ground point which is directly below both of them. Over the rest of the range of the pots, they work fine.
Thanks for any help,
Chris
Interestingly if the presence has recently 'buzzed', the volume control will stay silent, but if the amplifier has been left for a few minutes, moving the volume to zero will make noise. Can anybody offer any solutions to this problem because it is baffling me!
The only way these controls are connected is to a star ground point which is directly below both of them. Over the rest of the range of the pots, they work fine.
Thanks for any help,
Chris
like this........... TB-303 potentiometers
but it still only take a little oil or dust to lift the contacts
but it still only take a little oil or dust to lift the contacts
With regard to DC offset problems, wouldn't that gradually induce and increase buzzing or other problems as the pots were moved? In this case everything is silent to the point I thought the amp wasn't working and then a buzzing occurs only at the pot extreme. With an input signal the same happens except obviously the silence is replaced with music.
The problem is with VR16 (volume) and VR20 (presence). From what you said i think you might have read VR16 as VR10 and the grid stopper a solution I have considered a while ago for a previous similar problem so probably is worth a look. Thanks for the idea, I'll give it a try in addition to trying grounding the pot through a resistor.
Hi Chris
I tried modeling the phase-splitter / presence control in CircuitMaker. What a waste of time - got science fiction for bode plots. Meh. Happens sometimes.
Anyway, there's really slow time constants in there so if disturbed, it will take quite a while to settle down. (Hence "after a couple of minutes" in your first post).
Does the cathode current of the phase splitter have to go through the pot? If you rewire it as shown below, there won't be any DC across the pot, which should get rid of some of the weirdness. This will also get rid of whatever interaction there is between the feedback and the tail of the LTP.
A resistor (maybe 100k or so) between VR16 and C8 will probably sort out the problem with that pot - then the phase splitter never sees a source impedance less than 100k, even when the pot is all the way down.
Could maybe play with the value of C9? 100nF seems quite high, e.g. compared to C8. If it needs to be that high for stability, maybe it needs to be even higher?
I'm guessing the buzz you hear is a low-level oscillation, or does it sound like mains hum?
Anyway, Good luck!
Cheers - Godfrey
I tried modeling the phase-splitter / presence control in CircuitMaker. What a waste of time - got science fiction for bode plots. Meh. Happens sometimes.
Anyway, there's really slow time constants in there so if disturbed, it will take quite a while to settle down. (Hence "after a couple of minutes" in your first post).
Does the cathode current of the phase splitter have to go through the pot? If you rewire it as shown below, there won't be any DC across the pot, which should get rid of some of the weirdness. This will also get rid of whatever interaction there is between the feedback and the tail of the LTP.
A resistor (maybe 100k or so) between VR16 and C8 will probably sort out the problem with that pot - then the phase splitter never sees a source impedance less than 100k, even when the pot is all the way down.
Could maybe play with the value of C9? 100nF seems quite high, e.g. compared to C8. If it needs to be that high for stability, maybe it needs to be even higher?
I'm guessing the buzz you hear is a low-level oscillation, or does it sound like mains hum?
Anyway, Good luck!
Cheers - Godfrey
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Hi Godfrey,
Thanks for taking the time to look into this problem. I got the circuit for the presence control from aiken amps website and added it on as an afterthought really. The alternative you showed seems to be a reasonable alternative if it does the same job. I'm not sure to be honest whether the cathode current must go through the pot. I would have thought having the same network set up as you have drawn, the reactance of C46 will still operate on the high frequencies.
The 100nF C9 cap is just a decoupling cap so I generally designed my decoupling caps to pass all frequencies down to a few Hz. Haven't got the calculations to hand now but that capacitor should do the same - I'll check up on it.
The buzz does sound like some kind of low level oscillation and shows the hallmarks of some kind of ground loop. I did a bit of work for an hour this morning on it and noticed that when the input to the volume pot is touched with finger or screwdriver the humming stops. I have taken a twisted pair from the pot to attach a signal generator or mp3 player to for testing purposes and that's what I mean by the input! Might be worth putting a coaxial cable on instead of the twisted pair. I did check for any obvious problems such as braiding touching the chassis, but nothing really came to light. I did notice one braid was a little close to comfort to the chassis and moved it - when I moved it, a finger/screwdriver no longer stopped the humming.
Got a better idea of whats going on now I think anyway, so I'll get on the case again probably next week.
Thanks for taking the time to look into this problem. I got the circuit for the presence control from aiken amps website and added it on as an afterthought really. The alternative you showed seems to be a reasonable alternative if it does the same job. I'm not sure to be honest whether the cathode current must go through the pot. I would have thought having the same network set up as you have drawn, the reactance of C46 will still operate on the high frequencies.
The 100nF C9 cap is just a decoupling cap so I generally designed my decoupling caps to pass all frequencies down to a few Hz. Haven't got the calculations to hand now but that capacitor should do the same - I'll check up on it.
The buzz does sound like some kind of low level oscillation and shows the hallmarks of some kind of ground loop. I did a bit of work for an hour this morning on it and noticed that when the input to the volume pot is touched with finger or screwdriver the humming stops. I have taken a twisted pair from the pot to attach a signal generator or mp3 player to for testing purposes and that's what I mean by the input! Might be worth putting a coaxial cable on instead of the twisted pair. I did check for any obvious problems such as braiding touching the chassis, but nothing really came to light. I did notice one braid was a little close to comfort to the chassis and moved it - when I moved it, a finger/screwdriver no longer stopped the humming.
Got a better idea of whats going on now I think anyway, so I'll get on the case again probably next week.
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